Powdery Mildew: What Causes It and How to Get Rid of It
Powdery mildew is a fungal disease that shows as white, flour-like patches on the leaves. It is common in grow rooms and greenhouses, and far easier to keep out than to shift. Here is how to spot it, what brings it on, how to prevent it and what to do when you find it.
What powdery mildew looks like
- White or pale grey spots, usually on the upper side of a leaf. They rub off, then come back.
- The spots join into patches and spread to other leaves, stems and flowers.
- Badly affected leaves yellow, dry up and drop.
It often starts low down and inside the plant, where the leaves are shaded and the air moves least. Dried spray and hard-water marks look similar but do not spread.
What brings it on
The spores travel on the air and on clothes and tools, so most rooms meet them sooner or later. Unlike most moulds, powdery mildew does not need wet leaves. What helps it along:
- Still, humid air. Damp air that sits around the leaves helps the spores take hold. See our airflow guide.
- Crowded plants. Overlapping leaves trap damp air where no fan reaches.
- Big day-night swings. Warm air holds more water than cool air, so when the lights go off and the room cools, relative humidity climbs.
- New arrivals. It commonly comes in on cuttings and plants from elsewhere, so check them first.
Prevention: keep the humidity down
- Measure it with a hygrometer. The ThermoPro TP358 records the highest and lowest humidity over 24 hours and has high and low alarms.
- Set a target. Our airflow guide gives 55-70% relative humidity in vegetative growth and 40-55% in flowering. Check it on the VPD chart: its flowering range (1.0-1.5 kPa) works out at roughly 50-65% at 24°C, so about 50-55% in flowering sits inside both.
- Extract enough air, and keep the extractor running through the dark period, when humidity peaks. See how to size a fan and carbon filter.
- Add a dehumidifier when extraction cannot keep up: a sealed room, damp weather, big plants late in flowering.
- Soften the night-time drop with a little background heat: the Hylite Tube Heaters page says they help reduce mould and mildew by keeping the room drier.
| Dehumidifier | Removes up to | Good to know |
|---|---|---|
| RAM 12L | 12 litres a day | Holds a set humidity or runs continuously; 2-litre tank or continuous drain |
| RAM 20L | 20 litres a day at 30°C and 80% humidity; 10 at 27°C and 60% | 4.3-litre tank or continuous drain; maker's minimum room size is over 4 m² of floor |
| Quest 70 | 26 litres a day | Mounts overhead; restarts itself after a power cut |
| Quest 155 | 71 litres a day | As the Quest 70, for large spaces |
Which size? As a rule of thumb, plants give back to the air most of what you water, so a sealed room needs a unit that removes about that many litres a day, with some to spare; a room with an extractor needs less.
Prevention: keep the air moving
- Clip fans move air between and under the leaves. The Maxifan Clip Fan 200mm has an adjustable head; AC Infinity Cloudray fans come in 100, 150 and 225 mm.
- Add an oscillating wall fan. The WindKing Wall Fan swings side to side and up and down; its page says uniform conditions mean less chance of rot and powdery mildew.
- Give the plants room. Bendz clips bend stems apart and the ProNet 120 support net keeps plants separated.
What to do when you see it
- Check every plant, low down and inside.
- Take off the affected leaves. Bag them beside the plant so spores are not shaken over the rest, then wash your hands and scissors. A plant that is covered is better taken out of the room.
- Fix the air. Lower the humidity, point a fan through the plants and thin crowded growth, or the mildew is likely to return whatever you spray.
- Spray, if you choose to, covering both sides of every leaf. The Athena IPW page recommends a pump sprayer (the Garden HighPro Pumpro Sprayer 2L is one), with the fans off for even coverage. Wear the protective gear the label asks for, and turn the fans back on afterwards so the leaves dry.
- Keep checking every few days.
Sprays and additives compared
| Product | What its page says about mildew | How it is used |
|---|---|---|
| Athena IPW | Page says it controls fungal infections such as powdery mildew | Prevention 16-24 ml per litre, 2-3 times a week; treatment 24-32 ml per litre, starting at the lower dose. Spray with lights off |
| Optic Foliar Atak | Designed to protect against powdery mildew and other fungal pathogens; also assists with existing fungal issues | Ready to use, or a concentrate to dilute at the rate on the label; page says it can be sprayed under lights and leaves no residue |
| Grow Genius Whole In One | Enhances natural defences against fungal pathogens like powdery mildew; for use when symptoms appear or fortnightly as prevention | Foliar spray or root drench; typically 4-8 ml per litre for full-strength treatment |
| CannaCure | A bug repellent, mildew preventative and foliar feed in one bottle | Ready-to-use spray or concentrate |
| Buddhas Tree Solar Green Power | Added strength and protection against diseases such as powdery mildew | Silicon additive for the feed, also a foliar spray; 0.5 ml per litre (no separate spray rate given) |
Going by those pages, CannaCure and Solar Green Power are for prevention; the other three are also for use once mildew shows.
Whatever you use, read the label first: the dose, how often to repeat it, whether it can go on plants you'll eat, and how close to harvest you can use it (the Athena IPW page says up to week 5 of flowering). Late on, rely on removing leaves and drier air.
The Hotbox Sulfume uses sulphur, not a spray; its page says it is designed to stop the spread of fungal infections. As a rule, sulphur and oil-based sprays should not be used close together, as the combination can scorch leaves. Follow the full instructions supplied with the Sulfume. See all Pest Control.
Cleaning between crops
Mildew can carry over from one crop to the next, most easily on living plants, so check any mother plants or cuttings you are keeping, and clean while the room is empty.
- Clear out old leaves, roots and growing medium.
- Wash pots, trays and tools, and wipe down walls, floor and fan blades (see cleaning products). Silver Bullet Mist is a stabilised silver and hydrogen peroxide disinfectant; its page says it eliminates bacteria, viruses and fungi on contact.
Quick answers
What causes powdery mildew?
A fungus. Its spores arrive on the air or on new plants and take hold in still, humid air among crowded leaves.
How do I get rid of powdery mildew?
Take off the affected leaves and bag them, lower the humidity and get air moving through the plants. A spray used as its label directs can help, but mildew tends to come back while the room stays damp and still. Clean the room before the next crop.
Is powdery mildew the same as grey mould?
No. Powdery mildew is a dry white coating on the leaf; grey mould (botrytis) is a fuzzy grey-brown rot, often in dense flowers. Both like still, humid air.
More Growers' Guides
Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.